Can rapid loss and high variability of Martian methane be explained by surface H 2O 2?

Abstract

It has been reported by several groups that methane in the Martian atmosphere is both spatially and temporally variable. Gough et al. (2010) suggested that temperature dependent, reversible physical adsorption of methane onto Martian soils could explain this variability. However, it is also useful to consider if there might be chemical destruction of methane (and compensating sources) operating on seasonal time scales. The lifetime of Martian methane due to known chemical loss processes is long (on the order of hundreds of years). However, observations constrain the lifetime to be 4 years or less, and general circulation models suggest methane destruction must occur even faster (<1 year) to cause the reported variability and rapid disappearance. The Martian surface is known to be highly oxidizing based on the Viking Labeled Release experiments in which organic compounds were quickly oxidized by samples of the regolith. Here we test if simulated Martian soil is also oxidizing towards methane to determine if this is a relevant loss pathway for Martian methane. We find that although two of the analog surfaces studied, TiO 2·H 2O 2 and JSC-Mars-1 with H 2O 2, were able to oxidize the complex organic compounds (sugars and amino acids) used in the Viking Labeled Release experiments, these analogs were unable to oxidize methane to carbon dioxide within a 72 h experiment. Sodium and magnesium perchlorate, salts that were recently discovered at the Phoenix landing site and are potential strong oxidants, were not observed to directly oxidize either the organic solution or methane. The upper limit reaction coefficient, α, was found to be <4×10 −17 for methane loss on TiO 2·H 2O 2 and <2×10 −17 for methane loss on JSC-Mars-1 with H 2O 2. Unless the depth of soil on Mars that contains H 2O 2 is very deep (thicker than 500 m), the lifetime of methane with respect to heterogeneous oxidation by H 2O 2 is probably greater than 4 years. Therefore, reaction of methane with H 2O 2 on Martian soils does not appear to be a significant methane sink, and would not destroy methane rapidly enough to cause the reported atmospheric methane variability.